It is important that the movements of various units in a copying machine such as the optical scanning system, the image recording medium, the paper transport system and the like, should be accurately synchronized with one another to produce high quality copies. Generally, this synchronization is achieved by driving all the units or components of the copying machine by a common drive unit having a single transmission. Since, however, this necessitates an expensive transmission design, it is desirable to separate the drives for the various units so that the copying machine construction is simplified, the ease of servicing is increased and the inert masses required to be moved during the operation of the copying machine are reduced. The separate drives for the various units, however, have to be synchronized electronically. One way this can be achieved is by sensing the speed of one of the drive units, such as the motor for the image recording medium, and using it to generate a reference signal, which is used to control the speeds of the drives of the other units.
Separate drives for the various units necessitate controllers which can easily be adjusted at any time to the conditions of the control system such as the power of the drive motor, the inertia of driven parts and the like. The controllers also need to allow substantially immediate adjustment of the speed of the controlled motor or drive to the reference frequency within close tolerances.
Typically these controllers have a speed sensor for generating a speed signal the frequency of which represents the motor output speed, a proportional circuit which generates a proportional control (P-control) signal corresponding to the difference between the frequencies of the reference signal and of the speed signal, an integrator which generates an integral control (I-control) signal corresponding to the time integral of the difference between the frequencies of the reference signal and of the speed signal, an adder, which generates an output signal corresponding to the sum or the weighted sum of the two control signals (P-control and I-control), and an output stage for controlling the motor in accordance with the adder output signal. Controllers of this kind can be made using either analog or digital components.
Although analog controllers allow rapid signal processing and hence a low delay time for the control system, they have the disadvantages that the accuracy of the control is limited and the adjustment to the conditions of use at any time is a relatively laborious operation requiring the balancing of resistors and the like.
In contrast, digital control systems have high accuracy and little liability to trouble while being relatively cheap. In digital control systems, the input variables such as the frequency of the reference signal and the scanned actual speed signal are usually quantified and then arithmetic operations are carried out to form the set-value/actual-value difference used to generate the proportional component and the integral component, and to form the final control signal. Since these operations require some computer time, the control time is relatively long. It would be desirable, therefore, to have a digital control system which had a short control time.